If I am using Starter version and get to 100 submissions, can I seamlessly switch to Bronze level?

If I start a form in the Starter version, and use it to gather data, if/once I get up to 100 submissions can I still use the same form when upgrading to Bronze, or do I have to start over? 

Yes, you can switch between subscription levels at any time.  You can upgrade to Bronze when you need more submissions and downgrade back to a Free account when you do not.  If your submissions and storage total stay under the current account limits, your forms will be unaffected.